This invention relates to radiation imaging apparatus of the type employing an Anger-type scintillation camera, and coordinate computation circuitry for computing the coordinate of a light event from the camera signals.
An Anger-type scintillation camera, as disclosed in U.S. Pat. No. 3,011,057, includes a scintillation crystal responsive to radiation stimuli for producing light events, and a plurality of photomultipliers arranged in a predetermined array relative to the crystal, each of the photomultipliers having a photosensitive surface facing the crystal and being responsive to a light event for producing an output signal. The photomultipliers have overlapping fields of view and the position of a light event is ascertained by weighting each photomultiplier output in accordance with the distance of the photomultiplier from a reference axis, and combining the weighted outputs in a predetermined manner.
The accuracy with which the coordinates of a light event are computed depends on the coordinate computation circuitry which determines the weight applied to the photomultiplier signals and the manner in which they are combined. The resolution achieved with the coordinate computation circuitry disclosed in U.S. Pat. No. 3,011,057 is relatively poor because such circuitry does not take into account statistical variations in the number of photons produced by a light event, their distribution relative to the photocathodes of the photomultiplier tubes, and the efficiency of photomultipliers in converting photons to electrical signals.
U.S. Pat. No. 3,732,419 discloses that the accuracy of measurements can be improved by eliminating contributions from photomultipliers remote from a light event, or by enhancing contribution from photomultipliers near an event. The last-mentioned patent discloses a coordinate computation circuit that includes a circuit with a non-linear characteristic having the property of enhancing large signals more than small signals. Computational results are improved since the non-linear characteristic of the circuit essentially eliminates signal information with a relatively high probability of significant error which previously had exerted a strong effect on computational results.
The primary disadvantage of the coordinate computation circuitry disclosed in U.S. Pat. No. 3,732,419 is the reduction introduced by the non-linear characteristic circuit in output signal strength for the photomultipliers close to a light event in order to reduce the level of output signals for photomultipliers remote from a light event. Thus, position information useful in increasing the accuracy of the computations is reduced.
It is an object of the present invention to provide a new and improved radiation imaging apparatus which overcomes, or substantially reduces, the disadvantage of the prior art.